1. Field of the Invention
This invention relates to the water-proof structure of a front cover which forms a peripheral portion of a water-proof camera and more particularly to the water-proof structural arrangement of the window parts of the front cover.
2. Description of the Prior Art
The front cover of a water-proof camera which forms a peripheral portion of the camera has light transmissive parts or windows. These window parts are provided for a photo-taking optical system, a view-finder objective part, a counter, a flashlight emission part, etc. Heretofore, transparent plates made of a glass or plastic material have been used to form the windows for the optical system, the view-finder objective part, the counter, etc. and a protector such as a Fresnel member for the flashlight emission window with a packing seal material applied to these window parts to attain water-tightness. The conventional water-proof structural arrangement, however, not only increases the size of the camera but also results in a high cost.
A solution of this problem has been contrived as shown in FIG. 1 of the accompanying drawings. In this prior art contrivance, a circular recess 1b is provided on the outside of an aperture 1a formed in a front cover 1. An annular double-face adhesive tape 3 is applied to the bottom face of the recess 1b. Then, an outer window 2 made of a transparent material is fitted into the recess 1b. When the camera is placed underwater, pressure of 1 atm. is applied per 10 m of the depth of water to the front cover from outside in the direction of arrow A. Therefore, the outer window 2 would never be removed by this external pressure. However, the window 2 is vulnerable to internal pressure exerted from inside.
The internal pressure develops due to a rise in the inside temperature of the camera. Assuming that the back lid of the camera is closed when the inside temperature of the camera is at 0.degree. C. and the camera is left on the dash board of an automobile under direct rays of the sun, the inside temperature of the camera sometimes rises up to 60.degree. C. or above. Under such a condition, the inner pressure of the camera rises up to 1.2 atm. or above according to Boyle's and Charles' laws. In that event, differential pressure of about 0.2 atm. may develop across the outer window 2. Then, the differential pressure of 0.2 atm. comes to exert a force of about 0.2 kg-f/cm.sup.2 on the window 2. Assuming that the window 2 measures 1 cm.sup.2 in size, the window 2 is constantly under the force of 0.2 kg-f which is exerted in the direction of arrow B. In addition to that, the adhesive strength or power of the double-face adhesive tape 3 tends to be weakened by the above-stated temperature of 60.degree. C. Therefore, the prior art arrangement exposes the camera to a grave danger under such a condition.